One important application of wood-free coated fine paper is in the field of sheet-fed lithographic offset printing processes. There is a clear trend in this market towards shorter times to re-print and for converting in order to reduce the time of the production process and to facilitate handling.
Printers will have a clear advantage when paper can be almost instantly re-printed and converted (i.e. within 0.5 hours) as this is leading to a much higher efficiency of the process. Workflow in the printing industry today has been fully digitized, enabling same-day processing of a complete print-job (like e.g. CD-inserts), provided that the print-process in itself would enable to do so. The only component in the complete workflow that prevents speeding up of the full process is the interaction ink-paper, i.e. sufficient drying before converting. One can therefore say that ink drying time is the bottleneck or the rate determining step in the full sheet-fed lithographic offset printing process.
There is a belief that the shorter time to converting requires both an adequate physical drying component as well as a sufficient (but not necessarily 100% completed) chemical drying component of printed ink.
As supported by results of several studies, the physical drying component can e.g. be increased by adjustment of the porosity and/or of the surface energy of coating layers in a way that:                initial ink setting during residence time of the paper on the press is not too fast        ink setting is as fast as possible for ink setting times directly after printing.        
An induction period with respect to initial ink setting is necessary to avoid runnability problems on the press and to avoid loss in quality of the printed surface. Adjustment of surface energy appears also to be necessary to obtain superior print quality.
In the sheet-fed offset lithography printing process the quickset inks involved in general are mainly composed of ink colour pigment, at least partially unsaturated and/or conjugated resin, drying oil (which is an at least partially unsaturated and/or conjugated vegetable or biological oil) and a high boiling hydrocarbon (mineral) solvent (e.g. for adjusting the total flow characteristics). When printed on coated paper an initial physical absorption process starts, with adequately rapid rather selective penetration of the mineral oil phase into the paper coating and the raw paper-base. The residual resin- and drying oil rich phase precipitates due to the concomitant change of ink composition. It ends up with a relatively high viscosity and as a result it is (with ink colour pigment incorporated) more or less consolidated (often called “set”) on and somewhat in top of the coating surface, providing best conditions for optimal printing gloss properties.
In general such ‘set’ ink film is sufficiently rigid to withstand limited mechanical forces and enables the sheet to be re-printed on the second side of the sheet very soon after completing the first side. However its rigidity (especially wet rub resistance, abrasion resistance) normally has not well developed enough for ‘safe’ instantaneous further handling or converting (e.g. folding, cutting) of the printed paper without damaging printed images. In fact several hours up to a day or more might be needed before these next converting steps can be performed. In order to keep printing process economics viable, it is essential for printers to have this time interval minimised.
A well-known present method is to start up an additional chemical drying step of the printed ink layer, a so-called oxidative polymerisation or cross-linking reaction. Both the vegetable drying oil part, e.g. linseed oil, and the resin part are partly based upon (preferentially conjugated) unsaturated fatty acids. Oxygen in the air (or between the sheets in stack) adds to the double bonds of these fatty acids and resins to initially form hydroperoxides. After consecutive degradation of these hydroperoxides the resulting free radicals are very reactive. These radicals attack other fatty acid molecules and attach, forming new (larger) free radicals. This causes polymerisation to finally form a cross-linked ink network. The rate-determining step, formation and degradation of hydroperoxides, can appreciably be speeded up by the presence of special catalytic species (so-called primary/secondary/auxiliary driers or siccatives) in the ink. Possible is the addition of fatty acid salts (e.g. naphthenates or octoates) of transition type metals like cobalt to the ink prior to the printing. These catalysts are being added in small amounts to the printing inks, appreciably speeding up drying time from 100-200 h (non-catalysed situation) towards 1-10 h (catalysed situation). The complex mechanism of this ink cross-linking reaction path is visualised schematically in FIG. 1. This chemical drying can significantly improve resistance to mechanical forces.
Former catalytic drier systems in ink systems are similarly applied in e.g. commercial alkyd resin, solvent-based paints, also to speed up their chemical drying behaviour and to provide them with consumer-friendly behaviour. Latest developments on the paint market are water-based paint systems. In order to also speed up their chemical drying behaviour after application, specially adapted water-dispersible catalytic drier systems have been developed. In fact known primary/secondary/auxiliary drier systems have been modified with dedicated emulsifier combinations to make them sufficiently water-dispersible.
The present regular working method of printers therefore is to apply commercial inks with included catalytic drier systems and/or to add so-called drier systems to the ink prior to the printing to further speed up chemical drying. This however has several drawbacks. For instance a practical point is the appreciable reduction of the so-called ‘open time’ of the ink system, requesting a printer to clean-up the printing machine at the end of every regular 8 h working day cycle, or toxic anti-skinning agents like e.g. oximes have to be added to the ink. Another drawback is that a printer is forced to deal not only with standard ink but also to use several types of (more expensive) printing inks with an added drier system, depending on the absorptive and other printing properties of respective paper qualities.